When gas accumulations or other attenuation anomalies exist at shallow depths, they produce two detrimental effects on seismic data. One is the amplitude loss due to attenuation, and the other is wavefront distortion due to severe lateral velocity contrasts. As a result, the amplitudes of reflections from subsurface horizons below gas clouds or other anomalies are usually very low, making it difficult to identify such reflections. Such amplitude attenuation also diminishes or destroys the value of seismic amplitudes as an attribute for estimating reservoir properties.
One way to mitigate the problem of low amplitude is to apply automatic gain control (AGC) that almost equalizes the reflection amplitude. Although AGC is a powerful tool for revealing attenuated seismic reflections particularly when the reflection amplitude varies significantly), AGC destroys the amplitude integrity of the reflections thereby making attribute analysis meaningless.
Another class of amplitude compensation methods is based on a priori information. For example, if the reflection coefficient of a particular horizon below a shallow anomaly is known to be constant, but the reflection amplitudes from the horizon vary due to the anomaly, then one can generate scale factors that will force the amplitudes of reflections from the horizon to be constant and apply them to the seismic traces to correct the amplitudes of reflections from other horizons affected by the anomaly. However, this approach requires a priori knowledge about the reflection coefficient of a particular horizon. Otherwise, the method will blindly make the reflection amplitudes constant along the horizons, making the amplitudes unsuitable for amplitude analysis.
Without invoking any a priori assumptions about the amplitudes of reflections from a horizon, Brzostowski and McMechan discuss a way of estimating near-surface attenuation that could be used to mitigate amplitude variations caused by near-surface heterogeneities. (Brzostowski, M. A. and McMechan, G. A., "3-D tomographic imaging of near-surface seismic velocity and attenuation," Geophysics, 57, 393-406 (1992)). Unfortunately, their method requires a near-surface velocity profile to estimate the attenuation. Also, the method would be unstable if the ratio of observations to unknowns is low, which usually will be the case if one attempts to determine the attenuation for each cell for a near-surface volume. The authors qualitatively compare their attenuation model to near-surface geologic features, but the accuracy and resolution are insufficient to be used to correct any amplitude variations due to the near-surface heterogeneities.
Thus, there is a need for a method for determining surface-consistent attenuation scale factors without requiring either a near-surface velocity profile or any a priori assumptions about the amplitudes of reflections from horizons below shallow attenuation anomalies. The present invention satisfies this need.